1. Field of the Invention
The present invention is related to a power converting apparatus such as a voltage type inverter for driving an AC motor. More specifically, the present invention is directed to a power converting apparatus having a shortcircuit protection apparatus for protecting a shortcircuit of a self dis-igniting element (for instance, a bipolar transistor, and IGBT) functioning as a switching semiconductor element thereof.
2. Description of the Related Art
FIG. 6 is a diagram for showing the power converting apparatus including the conventional shortcircuit protection apparatus, which is described in Japanese Laid-open (KOKAI DISCLOSURE) Patent Application No. 5-219752. In this drawing, reference numeral 50 indicates a three-phase inverter circuit functioning as an inverter, reference numeral 52 (52a to 52f) shows a drive circuit for driving each of self dis-igniting elements 51a to 51f, a symbol "CT (CT1, CT2)" denotes a current detector. Also, reference numeral 55 indicates a control apparatus, reference numeral 56 shows an induction motor, symbol "Ed" indicates a DC power supply, reference number 60 shows a shortcircuit sensing circuit, reference numeral 61 denotes a voltage detecting circuit, reference numeral 62 represents a failure (malfunction) judging circuit, and reference numeral 63 indicates a failure signal.
In general, as the control system of the 3-phase inverter circuit 50, the PWM control is used by which the igniting signals (gate signals) are supplied to the self dis-igniting elements 51a to 51f for constituting the inverter so as to turn ON/OFF the self dis-igniting elements 51a to 51f, so that the magnitude of the output voltage of the inverter and the frequency are controlled.
FIGS. 7A to 7C are diagrams for explaining the arrangement of the shortcircuit sensing circuit 60 and the operations thereof as the shortcircuit protection apparatus of the conventional power converting apparatus. In this drawing, FIG. 7A shows an arrangement for 1 phase, namely a U phase of 3 phases. Since other V phase and W phase are similar to this U phase, drawings and explanations thereof are omitted. FIG. 7B is a time chart for representing operations of the shortcircuit sensing circuit 60 during the normal operation. FIG. 7C is a time chart for indicating operations of the shortcircuit sensing circuit 60 during the abnormal operation.
In FIG. 7A, in the voltage detecting circuit 61, the output voltage of the inverter is detected via the voltage dividing resistors R1 and R2, while using the potential at the negative polarity N of the DC power supply Ed. The detection voltage Vo is inputted into the failure judging circuit 62. In the failure judging circuit 62, this detection voltage Vo is compared by the two comparators CP1 and CP2, and the failure is judged by the failure judging device 64 in response to the outputs from the comparators CP1 and CP2.
In this case, the reference value "a" of the comparator CP1 is set to such a value higher than, equal to 1/2 of the DC power supply voltage "Ed", and also lower than this DC power supply voltage "Ed". Then, this reference value "a" is compared with the detection voltage Vo by the comparator CP1. When the detection voltage Vo is higher than the reference value "a", it is judged that the self dis-igniting element on the positive polarity side (in this case, element 51a) is turned ON.
Also, the reference value "b" of the comparator CP2 is set to such a value higher than, or equal to 0V, and also lower than, or equal to 1/2 of the DC power supply voltage Ed. Then, this reference value "b" is compared with the detection voltage Vo by the comparator CP2. When the detection voltage Vo is lower than the reference value "b", it is judged that the self dis-igniting element on the negative polarity side (in this case, element 51b) is turned ON.
Next, operations of the failure detector 64 will now be explained with reference to a time chart shown in FIGS. 7B and 7C. In this case, the outputs of the comparators CP1 and CP2 are expressed by the following logic levels (H, L):
detection value Vo&gt;reference value "a", or reference value "b"=L;
detection value Vo&lt;reference value "a", or reference value "b"=H.
During the normal operation of FIG. 7B, when the positive-polarity-sided switching element 51a is turned ON, the detection voltage Vo becomes higher than, or equal to the reference value "a" of the comparator CP1, and thus both the outputs from the comparators CP1 and CP2 become "L", namely are made coincident with each other. Similarly, in such a case that the negative-polarity-sided switching element 51d is turned ON during this normal operation, the detection value Vo is lower than, or equal to the reference value "b" of the comparator CP2, so that both the outputs from the comparators CP1 and CP2 become "H", namely are made coincident with each other.
However, as indicated in FIG. 7C, when a shortcircuit failure happens to occur at a time instant "t1", since the detection value Vo becomes lower than the reference value "a" of the comparator CP1 and higher than the reference value "b" of the comparator CP2, the output of the comparator CP1 becomes "H" and the output of the comparator CP2 becomes "L", so that the output of the comparator CP1 is not made coincident with the output of the comparator CP2. As explained above, since the output of the comparator CP1 is not made coincident with the output of the comparator CP2, it may be recognized that the inverter is brought into the shortcircuit condition.
Thus, the failure judging device 64 judges an occurrence of a failure based upon such a fact as to whether the outputs of the comparators CP1 and CP2 are made coincident, or incoincident with each other. When these outputs of the comparators CP1 and CP2 are not made coincident with each other, the failure judging device 64 transfers the failure signal 63 to the control apparatus 55.
Then, the control apparatus 55 performs the overcurrent protection operation, namely interrupts the gating operation of the self dis-igniting element 51.
FIG. 8 is a table for representing a relationship between gate signals of the conventional power converting apparatus and the switching operations of the switching semiconductor elements. In this table, symbol "GP" indicates a gate signal (positive polarity side), symbol "GN" denotes a gate signal (negative polarity side), symbol "Vo" represents an output voltage divided by the voltage dividing resistors R1, R2, and symbol "Io" shows an output current.
(1) In the case that the positive-polarity-sided gate signal GP is "ON" and the negative-polarity-sided gate signal GN is "OFF", the positive-polarity-sided switching element is turned ON and also the output voltage Vo becomes the positive polarity voltage.
(2) In the case that the positive-polarity-sided gate signal GP is "OFF " and the negative-polarity-sided gate signal GN is "ON", the negative-polarity-sided switching element is turned ON and also the output voltage Vo becomes the negative polarity voltage.
(3) In the case that the positive-polarity-sided gate signal GP and the negative-polarity-sided gate signal GN are "OFF", the output voltages Vo are different from each other, depending upon the output current Io:
(a) When the output current Io&gt;0, the output voltage Vo becomes the negative polarity voltage.
(b) When the output current Io&lt;0, the output voltage Vo becomes the positive polarity voltage.
(c) When the output current Io=0, the output voltage Vo becomes unstable.
FIGS. 9A and 9B are diagrams for indicating a relationship between the gate signals of the conventional power converting apparatus and the output voltage. FIG. 9A represents a relationship between the gate signal and the output voltage in the case of the output current Io&gt;0. FIG. 9B indicates a relationship between the gate signal and the output voltage in the case of the output current Io&lt;0. In FIGS. 9A and 9B, symbols "a1", "a2", "b1", and "b2" indicate time periods during which intermediate voltages between the positive polarity voltage and the negative polarity voltage, are produced as the output voltages when the gate signals are varied.
As represented in FIGS. 8 and 9A, 9B, in the above-described conventional power converting apparatus, when the upper arm of the 3-phase inverter circuit and the lower arm thereof are switched, there are certain possibilities that the potential could not be stable, depending upon the current flowing directions. The upper arms of the 3-phase inverter circuit corresponds to the self dis-igniting elements 51a, 51b, 51c, whereas the lower arm thereof corresponds to the self dis-igniting elements 51d, 51e, 51f. There is a problem such that the above possibilities may be detected as the failure, or the abnormal operation.
Also, in the case that the upper/lower shortcircuit happens to occur due to the failures of the self dis-igniting elements constituting the upper/lower arms, there is another problem in that no clear definition could be made of the damaged arm.